Conventional network-based communication systems include systems configured to operate in accordance with well-known synchronous transport standards, such as the synchronous optical network (SONET) and synchronous digital hierarchy (SDH) standards.
The SONET standard was developed by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI), and is described in the document ANSI T1.105-1988, entitled “American National Standard for Telecommunications—Digital Hierarchy Optical Interface Rates and Formats Specification” (September 1988), which is incorporated by reference herein. SDH is a corresponding standard developed by the International Telecommunication Union (ITU), set forth in ITU standards documents G.707 and G.708, which are incorporated by reference herein.
The basic unit of transmission in the SONET standard is referred to as synchronous transport signal level-1 (STS1). It has a serial transmission rate of 51.84 Megabits per second (Mbps). The corresponding unit in the SDH standard is referred to as synchronous transport module level-0 (STM0). Synchronous transport signals at higher levels comprise multiple STS1 or STM0 signals. For example, an intermediate unit of transmission in the SONET standard is referred to as synchronous transport signal level-3 (STS3). It has a serial transmission rate of 155.52 Mbps. The corresponding unit in the SDH standard is referred to as STM1.
A given STS3 or STM1 signal is organized in frames having a duration of 125 microseconds, each of which may be viewed as comprising nine rows by 270 columns of bytes, for a total frame capacity of 2,430 bytes per frame. The first nine bytes of each row comprise transport overhead (TOH), while the remaining 261 bytes of each row are referred to as a synchronous payload envelope (SPE). Synchronous transport via SONET or SDH generally involves a hierarchical arrangement in which an end-to-end path may comprise multiple lines with each line comprising multiple sections. The TOH includes section overhead (SOH), pointer information, and line overhead (LOH). The SPE includes path overhead (POH). Additional details regarding signal and frame formats can be found in the above-cited standards documents.
In conventional SONET or SDH network-based communication systems, synchronous transport signals like STS3 or STM1 are mapped to or from corresponding higher-rate optical signals such as a SONET OC-12 signal or an SDH STM4 signal. An OC-12 optical signal carries four STS3 signals, and thus has a rate of 622.08 Mbps. The SDH counterpart to the OC-12 signal is the STM4 signal, which carries four STM1 signals, and thus also has a rate of 622.08 Mbps. The mapping of these and other synchronous transport signals to or from higher-rate optical signals generally occurs in a physical layer device commonly referred to as a mapper, which may be used to implement an add-drop multiplexer (ADM) or other node of a SONET or SDH communication system.
Such a mapper typically interacts with a link layer processor. A link layer processor is one example of what is more generally referred to herein as a link layer device, where the term “link layer” generally denotes a switching function layer. Another example of a link layer device is a field programmable gate array (FPGA). These and other link layer devices can be used to implement processing associated with various packet-based protocols, such as Internet Protocol (IP) and Asynchronous Transfer Mode (ATM), as well as other protocols, such as Fiber Distributed Data Interface (FDDI). A given mapper or link layer device is often implemented in the form of an integrated circuit.
In many communication system applications, it is necessary to carry circuit-switched traffic such as T1/E1 traffic over a packet network such as an IP network or an ATM network. For example, T1/E1 traffic from a SONET/SDH network or other circuit-switched network may be carried using virtual containers (VCs) that are mapped to packets of an IP network or other packet network. Such VCs contain both POH and payload information. The first column of a given VC is used for the POH, and the remaining 260 columns are the payload, which may comprise one or more containers.
The mapping of VCs or other time-division multiplexed (TDM) data to IP packets or other types of packets may be performed in accordance with a circuit emulation protocol, such as the CEP protocol described in IETF RFC 4842, “Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) Circuit Emulation over Packet (CEP),” April 2007, which is incorporated by reference herein.